1. Field of the Invention
The present invention relates to a sheet or board like multiple layers laminated polyolefin foam suitable as partitions for containers, sheets or core materials for construction industry and civil engineering or for industrial goods, goods-delivery boxes in packaging applications, materials for harvest boxes and the like. More specifically, the present invention relates to a multiple layers laminated polyolefin foam in which multiple polyolefin layers are laminated on at least one side of a polyolefin foam sheet by a coextrusion method.
2. Description of the Related Art
Sheet or board like polyolefin foams have been widely used for various containers such as food trays, lunch boxes, bowls, cups, goods-delivery boxes, harvest boxes, and the like.
However, the polyolefin foams that have been used for the above-described applications had an expansion ratio of less than 3 times, were heavy, and had poor processability. Recently, the polyolefin foams have been suggested in which a polyolefin sheet was laminated on a polyolefin foam with an expansion ratio of 3 to 20 times. Such foams were disclosed, for example, in Japanese Patent Applications Laid-open Nos. H10-748, H11-105132, and H11-277696. Japanese Patent Application Laid-open No. H10-748 disclosed a board-like foam in which a synthetic resin film with a tensile strength of no less than 10 kg/mm2 was laminated by a thermal lamination method on a board-like polyolefin foam with a density of 0.07 to 0.25 g/cm3. Furthermore, Japanese Patent Application Laid-open No. H11-105132 disclosed a multiple layers synthetic resin foam in which a resin layer containing an inorganic filler in an amount of no less than 10 wt. % was laminated on a polyolefin foam sheet with a density of 0.5 to 0.1 g/cm3 by a coextrusion method. Japanese Patent Application Laid-open No. H11-277696 disclosed a method for the manufacture of a foam in which a polyolefin is laminated on at least one side of a propylene foam by a coextrusion method, wherein a foam having a surface layer extruded to have a tubular shape is cut open to obtain a sheet-like foam and taken up under heating to a temperature no less than 20° C. below the thermal deformation temperature of the polyolefin constituting the surface layer; the application also disclosed a laminated foam sheet or board. However, those products did not always have satisfactory properties. Thus, bending strength was insufficient as compared with that of the foam with an expansion ratio of less than 3 times, sufficient rigidity could not be obtained, and adhesion between the foam and laminated resin was locally insufficient, causing local peeling.
Furthermore, in applications such various containers such as goods-delivery boxes or partitions for containers, and the like, it is required to provide a polyolefin foam with functional properties by laminating it with a resin layer having an antistatic agent, an electrically conductive additive, a fungicide or a colorant added thereto.
In order to provide a laminated foam with functional properties such as antistatic properties, an additive providing the functional properties such as antistatic properties is mixed with a polyolefin used for lamination. In such case, however, a laminated foam with sufficient rigidity often cannot be obtained. Accordingly, the present invention provides a multiple layers laminated polyolefin foam combining small weight with sufficient rigidity.
Furthermore, the present invention also provides a multiple layers laminated polyolefin foam in which multiple polyolefin layers are laminated on at least one side of a polyolefin foam by a coextrusion method, this multiple layers laminated polyolefin foam combining small weight with sufficient rigidity. Moreover, the present invention provides a multiple layers laminated polyolefin foam having excellent antistatic properties and combining small weight with sufficient rigidity.
The inventors have conducted an intensive study of the melt flow rate, etc. of polyolefins laminated on polyolefin foam by a coextrusion method and have discovered that the closed cell ratio of the foam is decreased and sufficient rigidity cannot be obtained when the melt flow rate of the polyolefin laminated by a coextrusion method is low, the thickness of the polyolefin is large, and the density of the laminated foam is small. However, this information led to a new problem. Thus, the melt flow rate of a polyolefin is unavoidably decreased by the addition of an additive providing the laminated polyolefin with functional properties with an object of providing the laminated foam with functional properties such as antistatic properties and the like, and even when a multiple layers laminated polyolefin foam combining small weight with high rigidity is attempted to be obtained by laminating a polyolefin with a large thickness by a coextrusion method, the closed cell ratio of the foam decreases and a sufficient rigidity cannot be obtained. To resolve this problem, the inventors have conducted additional intensive study which led to the conception of the present invention.